Reciprocating piston compressors typically employ piston rings as seals to reduce gas leakage during compression of a gas. Typically the seal is not perfect and, during the upstroke of the piston, some of the gas leaks from the cylinder chamber past the piston rings and into the crankcase. In the case of air compressors the leakage or blow-by gas is typically vented to the surrounding atmosphere from a ventilated crankcase without a significant adverse effect. In the case of precious, toxic or combustible gases, external leaks from the compressor are undesirable, and leakage gas is preferably recaptured.
Some conventional crankcases are sealed so that blow-by gas which leaks into the crankcase is ducted back to the cylinder intake valves. If the compressor is operated with the suction intake at an elevated pressure (relative to ambient), then the crankcase must be a pressurized crankcase, designed to operate and remain gas-tight at elevated internal pressures typically equal to or slightly greater than the suction pressure.
In conventional compressors, the drive motor is typically separate from the crankcase. Typically, the drive shaft for the compressor protrudes from the crankcase and may be directly coupled to the drive motor, or driven via a belt power transmission. The shaft protruding from the crankcase employs rotating shaft seals to prevent leakage of the gas being compressed and the lubrication oil from the crankcase. The crankcase typically acts as an oil reservoir. The oil provides lubrication and cooling for the main shaft bearings and connecting rod bearings. In addition, the rotating shaft seal is typically cooled and lubricated by the lubricating oil in the crankcase. Such lubricated, rotating shaft seals have demonstrated reliability and longevity even at crankcase pressures of 600 psig. However, with oil lubricated compressors small amounts of oil tend to become entrained or carried in the compressed gas stream discharged from the compressor.
For some applications, it may not be acceptable to have any oil present in the compressed gas stream delivered from the compressor. Such applications include food and medical applications. Also, in fuel cell power plants it is important that reactant streams delivered to the fuel cell stacks are not contaminated with traces of oil, as such impurities can cause damage to system components, in particular to the membrane electrode assemblies in solid polymer fuel cell stacks. Also, oil traces can adversely affect reactant processing equipment, such as for example reformation and selective oxidation apparatus and purification modules, through which the compressed gas stream is directed en route to the fuel cell stack. Thus, compression of reactant streams, such as for example natural gas, oxygen and hydrogen, for eventual downstream delivery to a fuel cell stack, should be accomplished without introducing traces of oil into the streams.
oilless compressors are known in which there is no oil anywhere in the compressor apparatus. Polytetrafluoroethylene piston rings, cast iron cylinders and greased and sealed roller bearings are typically employed in such compressors. However, unlubricated or dry running rotating shaft seals which operate reliably under pressurization without leakage are not readily available.
It is therefore desirable to provide an oilless gas compressor with a pressurizable crankcase and motor containment vessel, in which the need for perimeter rotating shaft seals is obviated.